The idea was to produce a means of exchange, independent of any central authority, that could be transferred electronically in a secure, verifiable and immutable way. The liquidity for altcoins against the US dollar is often very poor. You can, if you wish, exchange your bitcoin for other cryptoassets rather than for cash. The bitcoin protocol stipulates that 21 million bitcoins will exist at some point. On the other hand, you have bitcoin-the-protocol, a distributed network that maintains a ledger of balances of bitcoin-the-token. Note: at time of writing, the surge of interest in bitcoin trading is placing strain on most retail buy and sell operations, so a degree of patience and caution is recommended. Although a pool has a much larger chance of solving a block and winning the reward, that reward will be split between all the pool members.